Calculators
Five tools for checking bets with numbers rather than intuition: an odds-format converter, a margin and fair-odds calculator, a value/EV estimate with a Kelly hint, an accumulator calculator, and ROI. Everything works right here, without registration and without sending data anywhere.
Here are the practical tools the rest of the site's articles refer to. All calculations happen right in your browser: nothing is sent to a server, no registration is needed, no data is saved anywhere. These are calculators for checking bets with numbers — and the theory behind each is covered by the relevant articles, which are linked.
Odds converter
Converts odds between decimal, fractional, and American formats and shows the implied probability. Choose the input format and enter a value. More on formats — in the article on odds.
Format converter
Margin and fair odds
Calculates the bookmaker's margin from a line and converts it into fair (margin-free) odds. For totals and handicaps switch the mode to two outcomes. The theory — in the article on the margin.
Margin calculator
Bet value (value / EV) and Kelly
Compares your probability estimate with the odds and shows the edge, the expected value (EV), and the recommended bet size by Kelly. In detail — in the article what actually works.
Value and EV calculator
Accumulator calculator
Multiplies the odds of the events in an accumulator and shows the combined odds, the implied probability of hitting, and the payout. Enter the odds separated by commas. Why accumulators are expensive — in the articles on the margin and myths.
Accumulator probability and payout
ROI calculator
Shows the return of play over the distance: profit as a percentage of turnover. Enter how much in total was staked and how much in total came back as payouts. What ROI means and why it matters more than individual wins — in the article what actually works.
ROI calculator
If these tools helped you see that the numbers in betting most often add up against the player — they've done their job. Calculating matters more than guessing; and why calculations are needed at all and what they change is explained in the article what actually works.
Frequently asked questions
The same odds are written differently: the decimal format (2.50) is used in Europe, the fractional one (3/2) in the UK, the American one (+150) in the US. These aren't different probabilities but different notations for the same price. The converter is useful when you read foreign sources, analysis, or compare offers: it instantly translates any format into the one you're used to and also shows the implied probability, so you can see what chance of the event is built into the odds.
A fair (margin-free) coefficient is the price of an event without the bookmaker's built-in commission. Real odds are always lowered by the amount of the margin, so the sum of implied probabilities for a match exceeds 100%. The margin calculator removes this markup: it takes all the outcome odds, calculates the margin, and shows what the odds would be at a fair probability estimate. This helps you understand how generous or greedy a line is and what the real assessment of the chances is without commission.
The Kelly criterion determines the share of the bankroll to bet when the expectation is positive: the share equals your edge divided by the odds minus one. The value calculator does this automatically — you enter your probability estimate, the odds, and the bankroll, and it shows the recommended size. An important caveat: full Kelly is too aggressive, professionals use a fractional share (most often a quarter), and it works only with a real positive expectation. If there's no edge, the bet isn't justified at any size.
ROI (return on investment) in betting is profit divided by turnover, as a percentage. It shows the effectiveness of play over the distance better than the absolute amount of a win: an ROI of 3% means that for every $1,000 of turnover you earned $30 on average. Profitable professionals usually have an ROI of single-digit percentages, and even that is considered an excellent result. A negative ROI means you're in the red over the distance — which is what happens to the overwhelming majority of players because of the margin.